/*
 * Scala (https://www.scala-lang.org)
 *
 * Copyright EPFL and Lightbend, Inc. dba Akka
 *
 * Licensed under Apache License 2.0
 * (http://www.apache.org/licenses/LICENSE-2.0).
 *
 * See the NOTICE file distributed with this work for
 * additional information regarding copyright ownership.
 */

package scala.tools.nsc
package backend.jvm
package opt

import scala.annotation.switch
import scala.collection.immutable.IntMap
import scala.collection.mutable
import scala.jdk.CollectionConverters._
import scala.reflect.internal.util.NoPosition
import scala.tools.asm.Opcodes._
import scala.tools.asm.Type
import scala.tools.asm.tree._
import scala.tools.nsc.backend.jvm.BTypes.InternalName
import scala.tools.nsc.backend.jvm.BackendReporting._
import scala.tools.nsc.backend.jvm.analysis.{AsmAnalyzer, BackendUtils, ProdConsAnalyzer}
import scala.tools.nsc.backend.jvm.opt.BytecodeUtils._

abstract class ClosureOptimizer {
  val postProcessor: PostProcessor

  import postProcessor.{bTypes, bTypesFromClassfile, callGraph, byteCodeRepository, localOpt, inliner, backendUtils}
  import bTypes._
  import bTypesFromClassfile._
  import backendUtils._
  import callGraph._
  import coreBTypes._
  import frontendAccess.backendReporting

  import ClosureOptimizer._

  private object closureInitOrdering extends Ordering[ClosureInstantiation] {
    override def compare(x: ClosureInstantiation, y: ClosureInstantiation): Int = {
      if (x eq y) return 0

      val cls = x.ownerClass.internalName compareTo y.ownerClass.internalName
      if (cls != 0) return cls

      val mName = x.ownerMethod.name compareTo y.ownerMethod.name
      if (mName != 0) return mName

      val mDesc = x.ownerMethod.desc compareTo y.ownerMethod.desc
      if (mDesc != 0) return mDesc

      def pos(inst: ClosureInstantiation) = inst.ownerMethod.instructions.indexOf(inst.lambdaMetaFactoryCall.indy)
      pos(x) - pos(y)
    }
  }

  /**
   * If a closure is allocated and invoked within the same method, re-write the invocation to the
   * closure body method.
   *
   * Note that the closure body method (generated by delambdafy:method) takes additional parameters
   * for the values captured by the closure. The bytecode is transformed from
   *
   *   [generate captured values]
   *   [closure init, capturing values]
   *   [...]
   *   [load closure object]
   *   [generate closure invocation arguments]
   *   [invoke closure.apply]
   *
   * to
   *
   *   [generate captured values]
   *   [store captured values into new locals]
   *   [load the captured values from locals]    // a future optimization will eliminate the closure
   *   [closure init, capturing values]          // instantiation if the closure object becomes unused
   *   [...]
   *   [load closure object]
   *   [generate closure invocation arguments]
   *   [store argument values into new locals]
   *   [drop the closure object]
   *   [load captured values from locals]
   *   [load argument values from locals]
   *   [invoke the closure body method]
   *
   * @param methods The methods to check for rewrites. If not defined, check all methods with closure
   *                instantiations.
   * @return The changed methods. The order of the resulting sequence is deterministic.
   */
  def rewriteClosureApplyInvocations(methods: Option[Iterable[MethodNode]], inlinerState: mutable.Map[MethodNode, inliner.MethodInlinerState]): mutable.LinkedHashSet[MethodNode] = {

    // sort all closure invocations to rewrite to ensure bytecode stability
    val toRewrite = mutable.TreeMap.empty[ClosureInstantiation, mutable.ArrayBuffer[(MethodInsnNode, Int)]](closureInitOrdering)
    def addRewrite(init: ClosureInstantiation, invocation: MethodInsnNode, stackHeight: Int): Unit = {
      val callsites = toRewrite.getOrElseUpdate(init, mutable.ArrayBuffer.empty[(MethodInsnNode, Int)])
      callsites += ((invocation, stackHeight))
    }

    // the `toList` prevents modifying closureInstantiations while iterating it.
    // minimalRemoveUnreachableCode (called in the loop) removes elements
    val methodsToRewrite = methods.getOrElse(closureInstantiations.keysIterator.toList)

    // For each closure instantiation find callsites of the closure and add them to the toRewrite
    // buffer (cannot change a method's bytecode while still looking for further invocations to
    // rewrite, the frame indices of the ProdCons analysis would get out of date). If a callsite
    // cannot be rewritten, e.g., because the lambda body method is not accessible, issue a warning.
    for (method <- methodsToRewrite if AsmAnalyzer.sizeOKForBasicValue(method)) closureInstantiations.get(method) match {
      case Some(closureInitsBeforeDCE) if closureInitsBeforeDCE.nonEmpty =>
        val ownerClass = closureInitsBeforeDCE.head._2.ownerClass.internalName

        // Advanced ProdCons queries (initialProducersForValueAt) expect no unreachable code.
        localOpt.minimalRemoveUnreachableCode(method, ownerClass)

        if (AsmAnalyzer.sizeOKForSourceValue(method)) closureInstantiations.get(method) match {
          case Some(closureInits) =>
            // A lazy val to ensure the analysis only runs if necessary (the value is passed by name to `closureCallsites`)
            lazy val prodCons = new ProdConsAnalyzer(method, ownerClass)

            for (init <- closureInits.valuesIterator) closureCallsites(init, prodCons) foreach {
              case Left(warning) =>
                backendReporting.optimizerWarning(warning.pos, warning.toString, backendReporting.siteString(ownerClass, method.name))

              case Right((invocation, stackHeight)) =>
                addRewrite(init, invocation, stackHeight)
            }

          case _ =>
        }

      case _ =>
    }

    val changedMethods = mutable.LinkedHashSet.empty[MethodNode]
    var previousMethod: MethodNode = null

    for ((closureInit, invocations) <- toRewrite) {
      // Local variables that hold the captured values and the closure invocation arguments.
      val (localsForCapturedValues, argumentLocalsList) = localsForClosureRewrite(closureInit)
      for ((invocation, stackHeight) <- invocations)
        rewriteClosureApplyInvocation(closureInit, invocation, stackHeight, localsForCapturedValues, argumentLocalsList)

      // toInit is sorted by `closureInitOrdering`, so multiple closure inits within a method are next to each other
      if (closureInit.ownerMethod != previousMethod) {
        previousMethod = closureInit.ownerMethod
        changedMethods += previousMethod
        val state = inlinerState.getOrElseUpdate(previousMethod, new inliner.MethodInlinerState)
        state.inlineLog.logClosureRewrite(closureInit, invocations, invocations.headOption.flatMap(p => state.outerCallsite(p._1)))
      }
    }

    changedMethods
  }

  /**
   * Insert instructions to store the values captured by a closure instantiation into local variables,
   * and load the values back to the stack.
   *
   * Returns the list of locals holding those captured values, and a list of locals that should be
   * used at the closure invocation callsite to store the arguments passed to the closure invocation.
   */
  private def localsForClosureRewrite(closureInit: ClosureInstantiation): (LocalsList, LocalsList) = {
    val ownerMethod = closureInit.ownerMethod
    val captureLocals = storeCaptures(closureInit)

    // allocate locals for storing the arguments of the closure apply callsites.
    // if there are multiple callsites, the same locals are re-used.
    val argTypes = closureInit.lambdaMetaFactoryCall.samMethodType.getArgumentTypes
    val firstArgLocal = BackendUtils.maxLocals(ownerMethod)

    val argLocals = LocalsList.fromTypes(firstArgLocal, argTypes)
    ownerMethod.maxLocals = firstArgLocal + argLocals.size

    (captureLocals, argLocals)
  }

  /**
   * Find all callsites of a closure within the method where the closure is allocated.
   */
  private def closureCallsites(closureInit: ClosureInstantiation, prodCons: => ProdConsAnalyzer): List[Either[RewriteClosureApplyToClosureBodyFailed, (MethodInsnNode, Int)]] = {
    val ownerMethod = closureInit.ownerMethod
    val ownerClass = closureInit.ownerClass
    val lambdaBodyHandle = closureInit.lambdaMetaFactoryCall.implMethod

    ownerMethod.instructions.iterator.asScala.collect({
      case invocation: MethodInsnNode if isSamInvocation(invocation, closureInit, prodCons) =>
        // TODO: This is maybe over-cautious.
        // We are checking if the closure body method is accessible at the closure callsite.
        // If the closure allocation has access to the body method, then the callsite (in the same
        // method as the allocation) should have access too.
        val bodyAccessible: Either[OptimizerWarning, Boolean] = for {
          (bodyMethodNode, declClass) <- byteCodeRepository.methodNode(lambdaBodyHandle.getOwner, lambdaBodyHandle.getName, lambdaBodyHandle.getDesc): Either[OptimizerWarning, (MethodNode, InternalName)]
          isAccessible                <- inliner.memberIsAccessible(bodyMethodNode.access, classBTypeFromParsedClassfile(declClass), classBTypeFromParsedClassfile(lambdaBodyHandle.getOwner), ownerClass)
        } yield {
          isAccessible
        }

        def pos = callGraph.callsites(ownerMethod).get(invocation).map(_.callsitePosition).getOrElse(NoPosition)
        val stackSize: Either[RewriteClosureApplyToClosureBodyFailed, Int] = bodyAccessible match {
          case Left(w)      => Left(RewriteClosureAccessCheckFailed(pos, w))
          case Right(false) => Left(RewriteClosureIllegalAccess(pos, ownerClass.internalName))
          case _            => Right(prodCons.frameAt(invocation).getStackSize)
        }

        stackSize.map((invocation, _))
    }).toList
  }

  /**
   * Check whether `invocation` invokes the SAM of the IndyLambda `closureInit`.
   *
   * In addition to a perfect match, we also identify cases where a generic FunctionN is created
   * but the invocation is to a specialized variant apply$sp... Vice-versa, we also allow the
   * case where a specialized FunctionN$sp.. is created but the generic apply is invoked. In
   * these cases, the translation will introduce the necessary box / unbox invocations. Example:
   *
   *   val f: Int => Any = (x: Int) => 1
   *   f(10)
   *
   * The IndyLambda creates a specialized `JFunction1$mcII$sp`, whose SAM is `apply$mcII$sp(I)I`.
   * The invocation calls `apply(Object)Object`: the method name and type don't match.
   * We identify these cases, insert the necessary unbox operation for the arguments, and invoke
   * the `$anonfun(I)I` method.
   *
   * Tests in InlinerTest.optimizeSpecializedClosures. In that test, methods t4/t4a/t5/t8 show
   * examples where the parameters have to be unboxed because generic `apply` is called, but the
   * lambda body method takes primitive types.
   * The opposite case is in t9: a the specialized `apply$sp..` is invoked, but the lambda body
   * method takes boxed arguments, so we have to insert boxing operations.
   */
  private def isSamInvocation(invocation: MethodInsnNode, closureInit: ClosureInstantiation, prodCons: => ProdConsAnalyzer): Boolean = {
    val indy = closureInit.lambdaMetaFactoryCall.indy
    if (invocation.getOpcode == INVOKESTATIC) false
    else {
      def closureIsReceiver = {
        val invocationFrame = prodCons.frameAt(invocation)
        val receiverSlot = {
          val numArgs = Type.getArgumentTypes(invocation.desc).length
          invocationFrame.stackTop - numArgs
        }
        val receiverProducers = prodCons.initialProducersForValueAt(invocation, receiverSlot)
        receiverProducers.size == 1 && receiverProducers.head == indy
      }

      def isSpecializedVersion(specName: String, nonSpecName: String) = specName.startsWith(nonSpecName) && specializationSuffix.pattern.matcher(specName.substring(nonSpecName.length)).matches

      def sameOrSpecializedType(specTp: Type, nonSpecTp: Type) = {
        specTp == nonSpecTp || {
          val specDesc = specTp.getDescriptor
          val nonSpecDesc = nonSpecTp.getDescriptor
          specDesc.length == 1 && primitives.contains(specDesc) && nonSpecDesc == ObjectRef.descriptor
        }
      }

      def specializedDescMatches(specMethodDesc: String, nonSpecMethodDesc: String) = {
        val specArgs = Type.getArgumentTypes(specMethodDesc)
        val nonSpecArgs = Type.getArgumentTypes(nonSpecMethodDesc)
        specArgs.corresponds(nonSpecArgs)(sameOrSpecializedType) && sameOrSpecializedType(Type.getReturnType(specMethodDesc), Type.getReturnType(nonSpecMethodDesc))
      }

      def nameAndDescMatch = {
        val aName = invocation.name
        val bName = indy.name
        val aDesc = invocation.desc
        val bDesc = closureInit.lambdaMetaFactoryCall.samMethodType.getDescriptor
        if (aName == bName) aDesc == bDesc
        else if (isSpecializedVersion(aName, bName)) specializedDescMatches(aDesc, bDesc)
        else if (isSpecializedVersion(bName, aName)) specializedDescMatches(bDesc, aDesc)
        else false
      }

      nameAndDescMatch && closureIsReceiver // most expensive check last
    }
  }

  private def isPrimitiveType(asmType: Type) = {
    val sort = asmType.getSort
    Type.VOID <= sort && sort <= Type.DOUBLE
  }

  /**
   * The argument types of the lambda body method may differ in two ways from the argument types of
   * the closure member method that is invoked (and replaced by a call to the body).
   *   - The lambda body method may have more specific types than the invoked closure member, see
   *     comment in [[LambdaMetaFactoryCall.unapply]].
   *   - The invoked closure member might be a specialized variant of the SAM or vice-versa, see
   *     comment method [[isSamInvocation]].
   */
  private def adaptStoredArguments(closureInit: ClosureInstantiation, invocation: MethodInsnNode): Int => Option[AbstractInsnNode] = {
    val invokeDesc = invocation.desc
    // The lambda body method has additional parameters for captured values. Here we need to consider
    // only those parameters of the body method that correspond to lambda parameters. This happens
    // to be exactly LMF.instantiatedMethodType. In fact, `LambdaMetaFactoryCall.unapply` ensures
    // that the body method signature is exactly (capturedParams + instantiatedMethodType).
    val lambdaBodyMethodDescWithoutCaptures = closureInit.lambdaMetaFactoryCall.instantiatedMethodType.getDescriptor
    if (invokeDesc == lambdaBodyMethodDescWithoutCaptures) {
      _ => None
    } else {
      val invokeArgTypes = Type.getArgumentTypes(invokeDesc)
      val implMethodArgTypes = Type.getArgumentTypes(lambdaBodyMethodDescWithoutCaptures)
      val res = new Array[Option[AbstractInsnNode]](invokeArgTypes.length)
      for (i <- invokeArgTypes.indices) {
        if (invokeArgTypes(i) == implMethodArgTypes(i)) {
          res(i) = None
        } else if (isPrimitiveType(implMethodArgTypes(i)) && invokeArgTypes(i).getDescriptor == ObjectRef.descriptor) {
          res(i) = Some(getScalaUnbox(implMethodArgTypes(i)))
        } else if (isPrimitiveType(invokeArgTypes(i)) && implMethodArgTypes(i).getDescriptor == ObjectRef.descriptor) {
          res(i) = Some(getScalaBox(invokeArgTypes(i)))
        } else {
          assert(!isPrimitiveType(invokeArgTypes(i)), invokeArgTypes(i))
          assert(!isPrimitiveType(implMethodArgTypes(i)), implMethodArgTypes(i))
          // The comment in the unapply method of `LambdaMetaFactoryCall` explains why we have to introduce
          // casts for arguments that have different types in samMethodType and instantiatedMethodType.
          //
          // Note:
          //   - invokeArgTypes is the same as the argument types in the IndyLambda's samMethodType,
          //     this is ensured by the `isSamInvocation` filter in this file
          //   - implMethodArgTypes is the same as the arg types in the IndyLambda's instantiatedMethodType,
          //     this is ensured by the unapply method in LambdaMetaFactoryCall (file CallGraph)
          res(i) = Some(new TypeInsnNode(CHECKCAST, implMethodArgTypes(i).getInternalName))
        }
      }
      res
    }
  }

  private def rewriteClosureApplyInvocation(closureInit: ClosureInstantiation, invocation: MethodInsnNode, stackHeight: Int, localsForCapturedValues: LocalsList, argumentLocalsList: LocalsList): Unit = {
    val ownerMethod = closureInit.ownerMethod
    val lambdaBodyHandle = closureInit.lambdaMetaFactoryCall.implMethod

    // store arguments
    insertStoreOps(invocation, ownerMethod, argumentLocalsList, adaptStoredArguments(closureInit, invocation))

    // drop the closure from the stack
    ownerMethod.instructions.insertBefore(invocation, new InsnNode(POP))

    val isNew = lambdaBodyHandle.getTag == H_NEWINVOKESPECIAL

    if (isNew) {
      val insns = ownerMethod.instructions
      insns.insertBefore(invocation, new TypeInsnNode(NEW, lambdaBodyHandle.getOwner))
      insns.insertBefore(invocation, new InsnNode(DUP))
    }

    // load captured values and arguments
    insertLoadOps(invocation, ownerMethod, localsForCapturedValues)
    insertLoadOps(invocation, ownerMethod, argumentLocalsList)

    // update maxStack
    // One slot per value is correct for long / double, see comment in the `analysis` package object.
    val numCapturedValues = localsForCapturedValues.locals.length
    val invocationStackHeight = stackHeight + numCapturedValues - 1 + (if (isNew) 2 else 0) // -1 because the closure is gone
    if (invocationStackHeight > BackendUtils.maxStack(ownerMethod))
      ownerMethod.maxStack = invocationStackHeight

    // replace the callsite with a new call to the body method
    val bodyOpcode = (lambdaBodyHandle.getTag: @switch) match {
      case H_INVOKEVIRTUAL    => INVOKEVIRTUAL
      case H_INVOKESTATIC     => INVOKESTATIC
      case H_INVOKESPECIAL    => INVOKESPECIAL
      case H_INVOKEINTERFACE  => INVOKEINTERFACE
      case H_NEWINVOKESPECIAL => INVOKESPECIAL
    }
    val bodyInvocation = new MethodInsnNode(bodyOpcode, lambdaBodyHandle.getOwner, lambdaBodyHandle.getName, lambdaBodyHandle.getDesc, lambdaBodyHandle.isInterface)
    ownerMethod.instructions.insertBefore(invocation, bodyInvocation)

    if (!isNew) {
      val bodyReturnType = Type.getReturnType(lambdaBodyHandle.getDesc)
      val invocationReturnType = Type.getReturnType(invocation.desc)
      if (isPrimitiveType(invocationReturnType) && bodyReturnType.getDescriptor == ObjectRef.descriptor) {
        val op =
          if (invocationReturnType.getSort == Type.VOID) getPop(1)
          else getScalaUnbox(invocationReturnType)
        ownerMethod.instructions.insertBefore(invocation, op)
      } else if (isPrimitiveType(bodyReturnType) && invocationReturnType.getDescriptor == ObjectRef.descriptor) {
        val op =
          if (bodyReturnType.getSort == Type.VOID) getBoxedUnit
          else getScalaBox(bodyReturnType)
        ownerMethod.instructions.insertBefore(invocation, op)
      } else {
        // see comment of that method
        fixLoadedNothingOrNullValue(bodyReturnType, bodyInvocation, ownerMethod, bTypes)
      }
    }

    ownerMethod.instructions.remove(invocation)

    // update the call graph
    val originalCallsite = callGraph.removeCallsite(invocation, ownerMethod)

    // the method node is needed for building the call graph entry
    val bodyMethod = byteCodeRepository.methodNode(lambdaBodyHandle.getOwner, lambdaBodyHandle.getName, lambdaBodyHandle.getDesc)
    val sourceFilePath = byteCodeRepository.compilingClasses.get(lambdaBodyHandle.getOwner).map(_._2)
    val callee = bodyMethod.map({
      case (bodyMethodNode, bodyMethodDeclClass) =>
        val bodyDeclClassType = classBTypeFromParsedClassfile(bodyMethodDeclClass)
        Callee(
          callee = bodyMethodNode,
          calleeDeclarationClass = bodyDeclClassType,
          isStaticallyResolved = true,
          sourceFilePath = sourceFilePath,
          annotatedInline = false,
          annotatedNoInline = false,
          samParamTypes = callGraph.samParamTypes(bodyMethodNode, Type.getArgumentTypes(bodyMethodNode.desc), bodyDeclClassType),
          calleeInfoWarning = None)
    })
    val argInfos = closureInit.capturedArgInfos ++ originalCallsite.map(cs => cs.argInfos map {
      case (index, info) => (index + numCapturedValues, info)
    }).getOrElse(IntMap.empty)
    val bodyMethodCallsite = Callsite(
      callsiteInstruction = bodyInvocation,
      callsiteMethod = ownerMethod,
      callsiteClass = closureInit.ownerClass,
      callee = callee,
      argInfos = argInfos,
      callsiteStackHeight = invocationStackHeight,
      receiverKnownNotNull = true, // see below (*)
      callsitePosition = originalCallsite.map(_.callsitePosition).getOrElse(NoPosition),
      annotatedInline = false,
      annotatedNoInline = false
    )
    // (*) The documentation in class LambdaMetafactory says:
    //     "if implMethod corresponds to an instance method, the first capture argument
    //     (corresponding to the receiver) must be non-null"
    // Explanation: If the lambda body method is non-static, the receiver is a captured
    // value. It can only be captured within some instance method, so we know it's non-null.
    callGraph.addCallsite(bodyMethodCallsite)

    // Rewriting a closure invocation may render code unreachable. For example, the body method of
    // (x: T) => ??? has return type Nothing$, and an ATHROW is added (see fixLoadedNothingOrNullValue).
    BackendUtils.clearDceDone(ownerMethod)
  }

  /**
   * Stores the values captured by a closure creation into fresh local variables, and loads the
   * values back onto the stack. Returns the list of locals holding the captured values.
   */
  private def storeCaptures(closureInit: ClosureInstantiation): LocalsList = {
    val indy = closureInit.lambdaMetaFactoryCall.indy
    val capturedTypes = Type.getArgumentTypes(indy.desc)
    val firstCaptureLocal = BackendUtils.maxLocals(closureInit.ownerMethod)

    // This could be optimized: in many cases the captured values are produced by LOAD instructions.
    // If the variable is not modified within the method, we could avoid introducing yet another
    // local. On the other hand, further optimizations (copy propagation, remove unused locals) will
    // clean it up.

    val localsForCaptures = LocalsList.fromTypes(firstCaptureLocal, capturedTypes)
    closureInit.ownerMethod.maxLocals = firstCaptureLocal + localsForCaptures.size

    insertStoreOps(indy, closureInit.ownerMethod, localsForCaptures, _ => None)
    insertLoadOps(indy, closureInit.ownerMethod, localsForCaptures)

    localsForCaptures
  }

  /**
   * Insert store operations in front of the `before` instruction to copy stack values into the
   * locals denoted by `localsList`.
   *
   * The lowest stack value is stored in the head of the locals list, so the last local is stored first.
   */
  private def insertStoreOps(before: AbstractInsnNode, methodNode: MethodNode, localsList: LocalsList, beforeStore: Int => Option[AbstractInsnNode]) = {
    // The first instruction needs to store into the last local of the `localsList`.
    // To avoid reversing the list, we use `insert(previous)`.
    val previous = before.getPrevious
    def ins(op: AbstractInsnNode) = methodNode.instructions.insert(previous, op)
    for ((l, i) <- localsList.locals.zipWithIndex) {
      ins(new VarInsnNode(l.storeOpcode, l.local))
      beforeStore(i) foreach ins
    }
  }

  /**
   * Insert load operations in front of the `before` instruction to copy the local values denoted
   * by `localsList` onto the stack.
   *
   * The head of the locals list will be the lowest value on the stack, so the first local is loaded first.
   */
  private def insertLoadOps(before: AbstractInsnNode, methodNode: MethodNode, localsList: LocalsList) = {
    for (l <- localsList.locals) {
      val op = new VarInsnNode(l.loadOpcode, l.local)
      methodNode.instructions.insertBefore(before, op)
    }
  }

  /**
   * A list of local variables. Each local stores information about its type, see class [[Local]].
   */
  case class LocalsList(locals: List[Local]) {
    val size = locals.iterator.map(_.size).sum
  }

  object LocalsList {
    /**
     * A list of local variables starting at `firstLocal` that can hold values of the types in the
     * `types` parameter.
     *
     * For example, `fromTypes(3, Array(Int, Long, String))` returns
     *   Local(3, intOpOffset)  ::
     *   Local(4, longOpOffset) ::  // note that this local occupies two slots, the next is at 6
     *   Local(6, refOpOffset)  ::
     *   Nil
     */
    def fromTypes(firstLocal: Int, types: Array[Type]): LocalsList = {
      var sizeTwoOffset = 0
      val locals = List.from[Local](types.indices.iterator.map(i => {
        // The ASM method `type.getOpcode` returns the opcode for operating on a value of `type`.
        val offset = types(i).getOpcode(ILOAD) - ILOAD
        val local = Local(firstLocal + i + sizeTwoOffset, offset)
        if (local.size == 2) sizeTwoOffset += 1
        local
      }))
      LocalsList(locals)
    }
  }

  /**
   * Stores a local variable index the opcode offset required for operating on that variable.
   *
   * The xLOAD / xSTORE opcodes are in the following sequence: I, L, F, D, A, so the offset for
   * a local variable holding a reference (`A`) is 4. See also method `getOpcode` in [[scala.tools.asm.Type]].
   */
  case class Local(local: Int, opcodeOffset: Int) {
    def size = if (loadOpcode == LLOAD || loadOpcode == DLOAD) 2  else 1

    def loadOpcode = ILOAD + opcodeOffset
    def storeOpcode = ISTORE + opcodeOffset
  }
}

object ClosureOptimizer {
  val primitives = "BSIJCFDZV"
  val specializationSuffix = s"(\\$$mc[$primitives]+\\$$sp)".r
}
